Plain English Meaning
The book that comes with the airplane and tells the pilot how to operate it safely. It contains the rules, numbers, and procedures specific to that exact make and model.
Full Definition
The official manufacturer-produced document that contains the operating limitations, procedures, performance data, weight and balance information, and systems descriptions for a specific airplane. The Pilot's Operating Handbook (POH) is the manufacturer's standard format used by general aviation manufacturers, while the Airplane Flight Manual (AFM) is the FAA-approved document required to be carried in the aircraft. For most modern light airplanes, the POH and AFM are combined into a single document, which is why the two terms are commonly written together as POH/AFM.